INDIGO DYES. By far the most important of these is indigo itself, a vegetable dye obtained from a tropical plant cultivated in India since the earliest times. The sap of this plant, when fermented under conditions excluding oxygen, yields indigo white, a soluble material having the formula if the fermentation proceeds in the open air, indigo blue, is pro duced. This substance is a derivative of the base called indol, which occurs ready formed, in small quantities, in many animal and vegetable secretions. It can be prepared arti ficially from aniline and ehloraldehyde. When indigo was found to consist of two ludol mole cules joined together and oxidized, the clue for the production of artificial indigo was at hand. It has since been found that any benzene deriva tive having a nitrogenous group and a two carbon group in the 'ortho' position may give rise to the formation of indigo. The first prac tical method, devised by Baeyer in 1880, involved the action of potassium hydroxide on ortho nitropropiolie avid; but many other methods have been devised since then, such as the action of melted potassium hydroxide on bromace tanilid. the action of halogenated acetone on
aniline, etc. Indigo is one of the most reliable dyestulTs, both as to brilliancy and permanency, and there is little difference in these respects between the natural and artificial products. The finished compound can, however. only be applied after reduction to the soluble indigo-white, and this makes its use in dyeing and printing some what cumbersome. In some of the methods for preparing artificial indigo, the fibre can be im pregnated with one ingredient and the other ap plied either in the dye-vat or from the printing rolls; consequently. indigo can be and is often directly prepared in the quantities and in the places in which it is needed. See INtnco.